A Diesel engine is conventionally equipped with an aftertreatment system that comprises an exhaust pipe, for leading the exhaust gas from the engine to the environment, and a plurality of aftertreatment devices located in the exhaust pipe, for degrading and/or removing pollutants from the exhaust gas before discharging it into the environment. In greater details, a conventional aftertreatment system generally comprises a Diesel Oxidation Catalyst (DOC), for oxidizing hydrocarbon (HC) and carbon monoxides (CO) into carbon dioxide (CO2) and water (H2O), and a Diesel Particulate Filter (DPF), located in the exhaust pipe downstream the DOC, for removing diesel particulate matter or soot from the exhaust gas.
In order to reduce NOx emissions, most aftertreatment systems further comprise a Selective Reduction Catalyst (SCR), which is located in the exhaust pipe downstream the DPF. The SCR is a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gas are converted into diatonic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent, typically ammonia (NH3), that is stored inside the catalyst. The ammonia is obtained through thermo-hydrolysis of a Diesel Exhaust Fluid (DEF), typically urea (CH4N2O), which is injected into the exhaust pipe through a dedicated injector located between the DPF and the SCR. The injection of DEF is controlled by an engine control unit (ECU) that determines the quantity of DEF to be injected in the exhaust pipe, in order to achieve an adequate NOx conversion rate inside the SCR, and then commands the injector accordingly.
Some ECU controls the DEF quantity to be injected according to a closed loop procedure, which is focused on the level of NH3 stored inside the SCR. In greater detail, this closed loop procedure provides for determining an index expressive of the NH3 storage level within the SCR, for determining a setpoint of said index, on the basis of the NOx concentration in the exhaust gas and of the exhaust gas temperature upstream the SCR, and for regulating the DEF quantity to be injected so as to minimize the difference between the index and the setpoint associated thereto. While getting an optimal NOx conversion efficiency of the SCR, this closed loop procedure sometimes involves an excessive DEF consumption, especially when the engine operates under heavy conditions, including operating conditions, such as for example high engine load and high engine speed, and environmental conditions, such as for example high environmental temperature or high altitude, i.e. low environmental pressure. Other ECU controls the DEF quantity to be injected according to an open loop procedure, which generally provides for calculating the DEF quantity as a function of the NOx concentration in the exhaust gas upstream the SCR. This open loop procedure normally gets an optimal DEF consumption but reduces the NOx conversion efficiency of the SCR.
In view of the above, at least one object is to optimize both NOx conversion efficiency and DEF consumption in every operating and environmental conditions, or at least in most of them. At least another object is to reach the above mentioned goal with a simple, rational and rather inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.